1. Stacks (on the Dock) sounded really cool until I went to use them. I'd much prefer that clicking on it opened the folder in Finder, I don't like the springboard or grid options.
2. I see that there are already hacks to axe the Shelf and 3D look to the Dock - I rather like it.
3. Make sure you uninstall UNO before upgrading to Leopard, whoops - live and learn.
4. I used to be able to connect to a Windows machine on my LAN by using it's network name, now I can't do that - am forced to connect to it via it's IP. And I'm having some issue connecting to certain shared folders with full read/write privileges. Not sure wtf is going on here.
5. The expanded use of CoverFlow is really nice.
6. Quick Look is awesome.
7. I'm using Time Machine, but don't have any experience with it to report just yet.
8. Dockless, a great util to keep some Apps from showing up in the Doc no longer works, this saddens me - hopefully it will be updated for Leopard soon.
9. Gizmo Project is not working in Leopard, I'm sure an update is not far off.
10. Spaces is nice, and there are some good things about it. But honestly, I miss some of the features from Desktop Manager, like being able to show the virtual desktops in the Menu Bar, complete with outlines of open Apps. It was a one click affair to change virtual desktops via mouse with Desktop Manager, with Spaces you have to click the icon on the menu bar and then click the desktop you want. This may sound trivial, but it's far more effort if you do it frequently.
11. Partitions can now be re-sized without destructively losing data in Leopard, wow, that's fantastic.
12. I used to use VineOSX VNC server because Apple Remote Desktop wouldn't allow me to connect with low bandwidth settings, and it doesn't work with Leopard. And unfortunately, the new ARD in Leopard also only works if I connect in high bandwidth mode.
13. Preview, I felt this was one of the unsung hero's of Tiger - and it's far improved for Leopard.
14. I'd swear that I read somewhere that Time Machine would give you the option of backing up USER data only, or everything including all system files. I see no such option available, however.
15. The Red, Yellow, and Green buttons on the top bar of windows are higher contrast now and much easier to distinguish.
16. Installs have more graphics when completed, easier to see.
17. iChat now has a tabbed interface, I just wish it were interoperable with other services.
18. Terminal is something I rarely use, but I can see that it's really beefed up. Not only is it tabbed - you can grab a tab and tear it off, move it back to being tabbed. Neat stuff for you command line junkies.
19. Mail.Appetizer no longer works with Mail, guess I'll move to a growl notification for mail.
20. The Menu Bar is better contrast now and easier to see items on it. Also, the Spotlight is no longer a garish blue, but a simple black icon.
21. The new Folder Icons are far better than Tiger.
22. Damn, have you seen Apple's stock price!

I'm not trying to be critical, I like Leopard - a lot. The itemized list above is just some of the observations I had. The connecting to local machine and mounting volumes is my really only issue I'm having right now. There's a lot of great stuff, and more stuff that I haven't had time to explore. But this is a worthy upgrade to Tiger.

My fingers are crossed that it's stable and runs as well as Tiger did.